Merge two TIFFs and save as new TIFF?

I have a conceptually simple task:

1. Read in two TIFF images
2. Overlay the second image onto the first
3. Save as a new TIFF

One catch: The first TIFF is 40,000 pixels by 30,000 pixels, 8-bit indexed color. Yes, I need a lot of memory. I'd like to get to that later. (The second TIFF is also 8-bit.)

I've found GDI+ (System.Drawing) to be incredibly restrictive. I cannot create a Graphics object from an 8-bit indexed color bitmap, for one thing (something that would help with memory).

So I am trying to learn WPF (System.Windows.Media.Imaging). I've been following several examples from MSDN and looking all around but nothing is working. Here's what I have so far:

Code:

---

        Dim workingDir As String = "C:\temp"
        Dim baseimg As String = "base.tif"
        Dim overlayimg As String = "overlay.tif"
        Dim outputFile As String = "output.tif"

        Dim dg As New DrawingGroup()
        Dim imgBase As New BitmapImage(New Uri(workingDir + "\" + baseimg))
        Dim imgOverlay As New BitmapImage(New Uri(workingDir + "\" + overlayimg))
        Dim rtb As New RenderTargetBitmap(200, 200, 200, 200, PixelFormats.Default)
        Dim dv As New DrawingVisual()
        Dim dc As DrawingContext = dv.RenderOpen()
        dc.DrawRectangle(
            New ImageBrush(imgBase),
            New Pen(Windows.Media.Brushes.Navy, 2),
            New Windows.Rect(10, 10, 100, 100)
        )
        rtb.Render(dv)

        Dim tif As New TiffBitmapEncoder()
        tif.Frames.Add(BitmapFrame.Create(rtb))
        Dim stm As System.IO.Stream = System.IO.File.Create(workingDir + "\" + outputFile)
        tif.Save(stm)
        stm.Dispose()

---

I think the saving part is working. Well, I get a TIFF image that is completely black (possibly transparent). It's as if rtb has nothing rendered in it.

Can someone lay out the basics for me here, or point me to a website that addresses this solution? Thanks.

Oh yeah, about memory. If I try to create rtb with 40,000x30,000, I get a parameter out of range exception or something. My hunch is it's a memory exception. (My 64-bit system has only 4GB.) Would streaming the files and merging the pixels individually be a better solution? With GDI+ there is LockBits and WriteableBitmap seems to be an option with WPF, but those two are beyond my skills at this moment. Help appreciated. Thanks!

Bump. Anyone here know WPF imaging?